Mr Zhang, chairman of the National People's Congress, China's ceremonial parliament, is the most senior Chinese official to visit since then-president Hu Jintao came in 2012.

The security, including officers keeping watch on a mountain peak far from the city centre, reflects official unease about possible disruption as Hong Kong's political atmosphere grows increasingly turbulent.

Discontent over Beijing's tightening grip on Hong Kong has risen since pro-democracy street protests rocked the city in late 2014, and calls for independence from radical political groups have become commonplace.

After arriving at the airport, Mr Zhang said he wanted "to relay warm greetings and best wishes" from Chinese president Xi Jinping and the central government "to the Hong Kong people".

Mr Zhang is scheduled to deliver a speech at a business conference on Wednesday morning. He is then expected to hold a rare meeting with four pro-democracy lawmakers and attend a dinner with Hong Kong's leader, Chief Executive Leung Chun-ying.

Around 6,000 police officers are on duty for Mr Zhang's visit, the South China Morning Post reported. Officers were even dispatched to the summit of Lion Rock, one of Hong Kong's tallest peaks, the Apple Daily newspaper said. However, they failed to prevent activists from unfurling a pro-democracy banner lower down the mountainside.

Police set up hundreds of plastic water-filled safety barriers on the streets surrounding the convention centre where Mr Zhang will speak and the hotel next door where he is staying.

In another sign of official nervousness over the visit, authorities glued together paving stones to prevent them from being ripped up and hurled by protesters. Pavement slabs were thrown during a February riot involving a pro-independence group that left 90 people injured.

Police in the neighbouring mainland city of Shenzhen arrested a Hong Kong man involved in a plot to use a drone to disrupt Mr Zhang's visit, China's official Xinhua news agency reported on Sunday.